No, the USB port is only intended to be used to connect a printer and therefore share the printer over the wireless network. If you plug any other USB device in the USB port, the Airport Express will simply not recognize the device.

Yes. You can set up a guest network. However, I have disabled mine simply due to the fact that guests are usually family members and I don't feel like they are any risk to my network. But it is easy to setup. During setup it gives you the option to create a guest network and you can set the attributes based on however much access or bandwidth you give your guests. In my case, if I use the guest option, the guests receive only G speed connection. The N is reserved for my own wireless stuff. Nowadays with the 3rd gen versions, if your ISP can deliver consistent GB speed >1gbps, you can reserve the N band for your guests and use the 802.11 ac for your own equip. Unfortunately for me, MY ISP is not capable of gigabit speed.

I have a MacBook Air with Airport Utility 6.3....
I want to connect to the internet when I move next week.
The ISP is called GreenLight in Wilson,NC which uses fiber optic cable.
Do I need to get only an Airport Express in order to connect using this service?

Deborah,
You will need an Airport Express, which acts as a Router (a very good Router!)
and you will also need a modem to connect to Greenlight.
Fiber Optic is new, state of the art, and a cable tv modem or a telephone modem won't work; your easiest solution is to allow GreenLight to provide/rent a fiber optic modem to you.
(Everybody needs two boxes, whatever the system.)
Enjoy your fiber optic; I'm jealous!

At this time I have Frontier internet. The internet cant be any SLOWER, the only reason I chose Directv is for the great deal on all the movie channels. When this plugs into my modem will it raise my bill?

I have a Mac and Comcast. I already own an Airport Extreme which is connected to my comcast router. I am looking to boost the wifi signal so that I can have it reach my back patio for parties. How would I set all of this up if it is even possible. If this doesn't work are there any other suggestion for what I can purchase to boost the signal from my extreme?

Yes. This is exactly what I am doing. I have an airport extreme in the main house. It does a good job covering the main house, but on the edges of the house and in the guest house signal is too weak. I added an express to the wired network in the guest house and configured it to extend the network. It isn't functioning as a repeater, rather an extender, so it doesn't slow my wireless network down, which repeaters do. In this case, both the extreme and the express use the same SID, but when the signal from the express is stronger the connecting device switches to it transparently and then its traffic goes through its connection back to the central switch. Very happy with this configuration. Totally transparent to my wifi devices. Strong signal everywhere.

Let me start by pointing out that Airpot Express does not currently support 802.11ac. This model supports 802.11a/b/g/n protocols. That being said, and to answer your question, yes you'll need a new computer the take advantage of 802.11ac. Not all new computers support ac yet though. For example, at the time of writing, the only Apple computers to have this capability are the new Mid 2013 MacBook Air.